Hyaluronic acid is a natural, linear polysaccharide constituted by a repetitive monomeric unit formed by d-glucuronic acid and N-acetylglucosamine. It is to be found in almost all the soft tissues of higher organisms and plays a vital role in many biological processes linked with the repair and recovery of tissue function.
Hyaluronic acid derivatives and their transformation into biomaterials such as, for example, membranes, nonwoven felts, meshes and sponges, for use in the biomedical and surgical fields, have been amply described in patent No. EP 0216453 B1. The key property of these products is their biodegradability, because once they have been implanted in the application site, they favour the release of native hyaluronic acid that is able to exercise its own biological functions (D. Campoccia et al., Biomaterials, vol. 19 (1998), page 2101).
Among other applications for ester derivatives of hyaluronic acid, EP 0652778 B1 claims the preparation of guide channels constituted by a combination of meshes immersed in a continuous polysaccharide matrix. Although these devices seem in some way to favour the growth of neurites and axons in short sections of damaged nerves, they are not able fully to support the process of repairing nerve function because of their rapid degradation.
It is an accepted fact that electric charges carried by a conductive material play a decisive role in increasing neuronal length and thereby accelerating nerve regeneration. In order to obtain these results, past techniques have exploited electric and magnetic fields generated externally or by introducing electric current directly through a section of damaged nerve. Neuronal growth has been demonstrated using materials with piezoelectric properties, such as polyvinylene difluoride (PVDF), the effect of which is attributable to the transient surface charges generated as a result of repeated mechanical stress. (R. F. Valentini et al., biomaterials, vol. 13, page 183 (1992)). The application of exogenous electromagnetic fields in in vitro and in vivo studies has shown better migration of the neuronal cells, this effect probably being due to a combination of biological mechanisms that depend on the redistribution of cytoskeletal proteins, such as actin and other molecules, on modifications in their formation and their capacity for favouring electric signals between parts of damaged nerve.
Although all these systems have proved to be partially effective, the use of piezoelectric materials and external electromagnetic fields have strong limitations. Without outside stimulation, the former are only able to generate electric properties for a limited period only, often insufficient for the functions they are to perform. Electromagnetic fields, on the other hand, cannot be focused exclusively on the nerve area to be regenerated and this may have unwanted effects on the surrounding healthy tissues.
More recently, polypyrrole has been used as a polymer to conduct electric signals in the biomedical field. Some particular applications described in the literature suggest that polypyrrole is a suitable matrix for the release of neurotransmitters or to make biosensors in association with other polymers and as a biomaterial with electroconductive properties to be used in the sector of nerve regeneration (V. Shastri et al., WO 97/16545; C. Schmidt, Tissue Engineering, vol. 26, 1999, page 617; J. Collier et al., Biomed. Mater. Res., 50, 2000, page 574). The special biological function demonstrated by polypyrrole (enabling nerve cells to take and grow) seems to be closely linked with the state of oxidation and the degree of hydrophilia due to the association with a particular doping agent, that may be constituted by an inorganic salt or by an anionic polysaccharide or polymer.
G. Robila et al. (J. Of Applied Polymer Sci., vol. 66, 1997, page 591) describe the use of polyurethane sulphate as a doping agent for use in making composite biomaterials based on polypyrrole, while C. Schmidt et al. (Proc. Nat. Acad. Sci. USA, vol. 94, 1997, page 8948) present a series of results from in vitro and in vivo studies to determine the cell response induced by the implantation of a composite biomaterial constituted by polypyrrole in association with polystyrene sulphate as doping agent.
Among the hyaluronic acid derivatives with a marked anionic charge linked with the type of chemical modification conducted on the polymer chain, of particular interest are those obtained by substituting the hydroxy functions with sulphated groups. These composites have already been described by Barbucci et al (EP 0702699 B1), who detail the preparation of biopolymers with anticoagulant properties intended for biomedical applications where there is contact with the blood. Again for the preparation of biomaterials with haemocompatible properties, the same R. Barbucci describes, in patent application No. PCT WO99/43728, composites constituted by polyurethane chemically linked with sulphated hyaluronic acid derivatives.
Although the electropolymerisation reaction of pyrrole with HA and polystyrene sulphonate (PSS) is well known as a procedure of synthesis to obtain biocompatible polymer films, both in the form of a single layer (PPy/HA and PPy/PSS) and as multiple layers (“Synthesis and characterisation of polypyrrole-hyaluronic acid composite biomaterials for tissue engineering applications”, Collier et al., J. Biomed. Mater. Res., 50, 574-584, 2000; PCT Publication No. WO 97/16545), the inventive aspect that characterises the present invention consists in using esterified hyaluronic acid derivatives (HYAFF®), e.g. in the form of woven, non-woven fabrics and guide channels, as supporting materials on which to activate the polymerisation reaction between PPy and PSS, hyaluronic acid, HAOSO3, HYOXX™ and the derivatives thereof, obtaining composites that can be grafted directly into living organisms.